Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. The large variety of active sites in Ziegler-Natta catalysts makes it difficult to control polymer architecture. To improve polymer properties, single-site catalysts, in particular metallocenes, are beginning to replace Ziegler-Natta catalysts.
U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Unbridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No. 6,908,972) readily copolymerize α-olefins and provide polymers with varying levels of long-chain branching. Some of the examples have very low long-chain branching (e.g., Example 15 reports no long-chain branching and Mw=90,700). For a discussion of long-chain branching in polyethylene, see Macromolecules 39 (2006) 1474 and references cited therein.
Copending application Ser. No. 11/899,090, filed 4 Sep. 2007, discloses a slurry process to make an ethylene copolymer having, among other attributes, low long chain branching. Indenoindolyl complexes, including dialkylsilyl-bridged bis(indeno[1,2-b]indolyl)zirconium complexes, are used. Hydrogen is not required, and the effect of including hydrogen on polymer molecular weight distribution is not discussed. In the examples of the '090 application, the polymers have narrow molecular weight distributions.
Copending application Ser. No. 11/899,113, filed 4 Sep. 2007, discloses a high-activity slurry process to make an ethylene copolymer with broad molecular weight distribution with certain indenoindolyl complexes, including dialkylsilyl-bridged bis(indeno[1,2-b]indolyl)zirconium complexes. Several of the polymers made have Mw/Mn values greater than 10, but the reference is generally silent regarding the long-chain branching content of the polymers.
Despite the industry's considerable experience with single-site catalysts generally and indenoindolyl catalysts in particular, there is a need for improvement. Often, to get broad molecular weight distribution, polymer blends are prepared, or processes utilizing two or more reactors are used. Catalysts that provide good incorporation of α-olefins also frequently produce polyethylene with too much long-chain branching, which adversely impacts polymer properties. Many processes that provide good incorporation of α-olefins do not incorporate the comonomer uniformly. The comonomer is either distributed unevenly in the polyethylene chain or is predominantly in the low-molecular-weight fraction.
Usually, a compromise must be made among properties. A valuable process would employ a single reactor and would provide polyethylene with both broad molecular weight distribution and low long-chain branching. Ideally, the polyethylene would demonstrate good processing and physical properties in the production of blown films.